1. Field of the Invention
The present invention generally concerns radio antennas. The present invention particularly concerns vandalism-resistent radio antennas usable predominantly for radio frequencies above 30 Mhz during line-of-sight radio wave propagation. The present invention still more particularly concerns radio antennas usable with post-mounted electronic devices which, in addition to communicating by radio, also communicate by wire.
2. Description of the Prior Art
Many modern electronic devices that communicate by radio are situated in the out of doors, often in locations that are neither readily nor continuously subject to human supervision or surveillance. Such devices include, for example, emergency radio call boxes and radio-communicating irrigation controllers.
Emergency radio call boxes are commonly located in parks, on college campuses, or alongside roadways. The call boxes are typically situated on strong posts. They are typically battery or solar powered. They permit stranded motorists and other users to contact a central authority by radio. The antenna for the call box radio is typically located at a height well above the call box, sometimes even upon a separate structure from the post supporting the call box. When so situated high above the ground (12 feet plus), the antenna normally needs be supported on a pole or other support structure of considerable strength. The resistance offered by the call box to damage from vandalism is primarily physical, with the call box having considerable strength and durability. The antenna is, however, typically of such length, and aspect ratio, as precludes that it should be able to withstand being bent or broken under force of a vandal. Accordingly, protection of a call box antenna from vandalism has heretofore been accorded primarily by the inaccessibility of the antenna.
Nonetheless to the nominal vandalism resistance presented, the exposed antenna of a common radio call box is usually quite readily bent or broken--at least by a determined vandal. If the antenna of a call box is severely bent, or broken off, by a vandal then the function of the call box may be degraded, or completely interrupted, pending repairs. If an emergency requiring use of the call box occurs during this period then consequences of the vandalism may be magnified.
Irrigation controllers are another type of electronic device commonly located in the out of doors--typically proximately to irrigated areas--and communicating by radio. Radio-controlled irrigation controllers communicate by radio to a central station for the any of the purposes of control, coordination, or the reporting of information regarding local conditions and/or the historical conduct of irrigation. Radio-communicating irrigation controllers are typically mounted atop posts, and a convenient height above the ground (i.e., four or so feet) so as to readily permit the entrance of commands/or data, and/or the viewing of the operational status of the controller, by a programmer/maintainer of the irrigation system.
As well as communicating by radio, post-mounted irrigation controllers typically also communicate by wire with at least one, and typically several (e.g., 8-32) electric valves used to control the flow of irrigation water. The wires to these valves are typically routed from the irrigation controller through the tubular body of its support post and then underground until emerging at the site of above-ground valves (or into the vaults containing such valves as are positioned below ground). The irrigation controllers may also communicate by wire with above-ground or buried sensors, such as soil moisture sensors.
The antenna for a post-mounted radio-communicating irrigation controller is typically located above the irrigation controller, and uppermost upon the same post that supports the irrigation controller. The antenna is typically a straight rod similar to an automobile radio antenna. Unless raised to a position high above the ground by a costly extension to the post, or else made very strong--again at considerable cost--the antenna is high susceptible to vandalism. It may, in particular, be bent or broken off. If broken off, an antenna can be misused as a weapon in the manner of a sword. Should someone be hurt then the owner or manufacture of the irrigation controller and antenna could be subject to legal liability. If a broken-off antenna is discarded or disposed of in a non-environmentally sound manner, it constitutes a durable piece of metal that is neither readily environmentally degraded nor assimilated.
Despite the considerable attempts to make the external appearance of an irrigation controller aesthetically appealing, as is exemplified in U.S. design patent No. AAA,AAA for a Light-Powered Irrigation Control Head to the same inventor and the same assignee as is the present patent application, an irrigation controller is commonly located in an inconspicuous location. Moreover, the locations of irrigation, such as upon golf courses, are not always attended by humans. Because of strong economic considerations in the locating of irrigation controllers, any susceptibility of the controllers and/or their antennas to vandalism is not, and cannot readily be, a major consideration in site selection. Because modern irrigation controllers are, nonetheless to having the computational power of small computers and a value of several hundreds of dollars U.S. (circa 1993), commonly located in the out of doors at often unattended, and often obscure, locations, they must commonly be very strongly resistant to vandalism, including in any antenna component.
Accordingly, it would be useful if an antenna could be provided for post-mounted radio-communicating devices--specifically including radio call boxes and radio-communicating irrigation controllers--which antenna was both economical in cost and functionally effective for radio communication while also enjoying improved resistance to vandalism.
One relatively secure place for a radio antenna would be inside the same post that supports the radio-communicating device. Of course, in order to transmit radio waves from its interior the post cannot be made of metal or any other conductor, and must be made of an electrically non-conducting material such as wood or, particularly, fiberglass or plastic. With modern materials (circa 1993) this does not present any appreciable problem.
Alas, if wires are also present within this post, as they commonly are, then these wires are essentially at signal ground relative to radio frequencies, and to radio frequency signals. This is true whether or not the wires carry power or communication signals. Wires commonly exist within the tubular post supporting a roadside radio call box for the purpose of powering the call box from batteries located at, or proximate to, ground level. Wires commonly exist within the tubular post supporting an irrigation controller for the necessary purpose of connecting the irrigation controller to the electric valves, if not also to remotely-situated wire-connected soil sensors. Additionally, for those irrigation controllers that are powered by sources of power external to the controller (at its post top position), power wires are also typically present inside the tubular post.
Any such wires at radio frequency ground within a tubular post as were to be located proximately to, and typically randomly relative to, any radio frequency antenna that was to be co-located within the same tubular post would present a severe, and likely insurmountable and untenable, problem to the radiation field of the antenna. The radiation pattern, and the gain, of the antenna would be severely degraded, or destroyed, in all directions where a radio frequency ground wire was present. This condition, and the typical use of metal in the economical fabrication of strong tubular posts for the support of electronic devices, is why antennas are not commonly found inside such tubular posts.